Coupling unit



FIG. 1 is a top perspective view of a coupling unit showing my new design;

FIG. 2 is a bottom perspective view thereof;

FIG. 3 is a top perspective view of a modified form of the coupling unit shown in FIG. 1; and

FIG. 4 is a bottom perspective view of FIG. 3. 

The ornamental design for a coupling unit, as shown and described. 